How would you explain an atomic mass number?
Well it's the sum of the massive nuclear particles, protons and neutrons.
As you know, a nucleus contains a given number of nuclides: (i) protons, massive positively charged particles; and (ii) neutrons, massive neutrally charged particles. The associated electronic mass is fairly insignificant in comparison.
The number of protons gives
However the nucleus may contain different numbers of neutrons, which difference gives rise to isotopes. For most hydrogen nuclei, where
The weighted average of all isotopes is the mass number, and this number is quoted on the Periodic Table; of course you will never be expected to remember these numbers in an exam. You will be expected to utilize the Periodic Table provided to explain atomic numbers and mass numbers.